Make sure you have a folder shared on both PCs, all firewalls are turned off and the subnet mask on both PCs are 255.255.255.0. Make sure the first numbers of the IP address are the same (eg 192.168.0) and that the fourth number differs between the computers. Install the IPX/SPX protocol just in case (don't laugh, I've had many networks miraculously work because of this) and make sure the two PCs don't have the same machine names.
Oh yeah, and use Simple Sharing at first...
So I should use static IPs? Currently I use "Obtain IP address atomatically"